This invention relates to electronic copiers and, more particularly, to improvements in the charging device in the electronic copiers.
In the electronic copier, a light-sensitive medium is precharged by a precharger prior to the exposure thereof to a light image of an original as is well known in the part.
The prior-art prechargers include a corona charger making use of the corona discharge and a contact charging means which effects charging of light-sensitive with a conductive brush or a conductive roller in forced contact with the medium. The corona charger is simple in construction and has steady performance. However, it uses a high voltage source of 5 to 10 kv, and therefore it has drawbacks in safety and economy. Further, during the corona discharge ozone is produced and extremely deteriorates the properties of the light-sensitive medium, developer and other materials. The contact charging means requires a low voltage source of only 0.5 to 1 kV, and hence it is free from the generation of ozone. On the demerit side of this means, however, it is difficult to continuously obtain uniform charging, and the reliability is low. With this means, the voltage required for the charging of the light-sensitive medium is applied in one step in the form of a pulse, and a maximum voltage is applied at the point of commencement of contact between the light-sensitive medium and conductive brush or conductive roller. Therefore, a sharp charge of applied voltage occurs at the commencement of contact, and uniform charging of the light-sensitive medium cannot be obtained.
In addition, the light-sensitive medium is liable to be electrically damaged.